How To Remove Underbody Sealer (Tar)?

[oilypaws]

I've now got my Morrie (minus every part which can be removed) sitting on its side on a wheeled frame so I can work on the underside.

It is coated in some kind of underbody seal which is basically thick tar. About 80% of this seems sound (i.e not coming off...) but some is flaking. Under the seal I can see bare metal (with surface rust) in places. Other places where I have scraped I found original paint.

There is no way it will come off with a wire-brush on the grinder - it just melts and spreads around. I tried using a hot air gun and a scraper, but there are problems: It leaves a thin film of tar which is melted on to the metal and still won't come off (tried scraping / wire brush / kerosene / hot kero / hot air + kero / hot air + kero + wire brush + rag). I end up with a black coating on the (slightly rusty) surface which I think will be impervious to rust treatments and un-sticky for paint. Also, it is really slow - I'll be scraping for a year at the rate I am going.

Here are some possibilities:

-Sand blasting: Will this take off tar? (I doubt it, at least not the bulk without first scraping it). And is it true that this needs to be done professionally? (I have a small compressor...). I could scrape first, then get it sand-blasted...

-Toxic chemicals: Is there anything which helps, how fast will it kill me, and will it turn my shed in to a contaminated land site?

Any suggestions?

Here is a radical idea: What if I leave the bulk of it, and just scrape the loose bits? I would then rust-treat them if they were tar free and rusty, and then just paint everything with more tarry stuff... It seems silly to scrape off a lot of perfectly good sealer. The problem is just how to deal with the problem areas.

I am after something functional, and appearance is not critical (it's underneath, after all). I don't think any of the job is going to come out better-than-original since I don't have the skills or patience (not yet). I am in awe of the beautiful jobs I have seen. Anyway, I really want it to be functional, OK looking and not rust any more (for a few years anyway).

Remember we don't use salt on the roads over here, so the underside of the car generally doesn't rust very much. The chassis and doors usually rust from the inside where dampness sits.

[chickenjohn]

Try using a scraper with and without a heat gun. Sometimes, the underseal just flakes off with the scraper used cold as underseal is quite brittle this time of year in an unheated garage.

For more stubborn stuff, apply heat from hot air gun (not so much as to scrorch as you have done) but just enough to warm and loosen, then scrape off.

Finally, to remove the residue, brush on some parrafin, leave to soak in for a few mins and the underseal remains will just wipe off onto a rag. Any final bits will come off with light hand sanding with 40grit aluminium oxide paper. Then wipe with panel wipe before further painting.

[taupe]

Hi

Ive just taken all of this from the underside of my traveller. I waited for a very frosty night and attacked it with a slightly blunt 1" wood chisel. It came off a treat (Really clean) in big flakes mostly and was the original bitumen type underseal as you describe. The whole underside took about 4-5 hours. If the chisel is too sharp it tends to dig in to the metal and leaves scratch marks all over.

I had tried with heat first and had the same problem as you. Used thinners and petrol to remove but it took ages for a small area and left a stain in any surface rust patches.

Taupe

[tortron]

go to your local industrial welding supply place (or even a fish market) and get a bin of dry Ice.

Wearing thick welding gloves put a handful of ice in a bag (cloth is good)
hold the ice against the undercoating for about 20 seconds.
This will make it very brittle and it will come off very easily with a scraper. You will be done in less than half the time with just the scraper - tried and tested

[tortron]

i see you are NZ
got to BOC - thats where we get ours

[oilypaws]

Thanks for suggestions - someone mentioned dry ice but I had no idea where to get it. Will have to try!

[Bazzalucas]

I think this question is also covered in the thread called, "Just being lazy".